1. Field of the Invention
This invention relates to a method of manufacturing an avalanche photodiode and particularly to a method of manufacturing an avalanche photodiode capable of effectively preventing edge breakdown.
2. Description of the Related Art
Light receiving device is used as an essential element in optical communication systems and serves as converting a photo signal transferred via a optical fiber to an electrical signal. Among semiconductor device as such a light receiving device, there is a photodiode and among III-V compound semiconductors there are p-i-n photodiode and an avalanche photodiode (hereinafter "APD").
Generally, in APD, carriers generated by incident light collide with each other via a collision ionization and thus another carrier is generated.
However, in APD, as an edge portion curvature of a pn junction is smaller than a center portion curvature thereof, the electric field of the edge portion becomes higher than that of the center portion due to a very high reverse voltage applied in order to generate an avalanche breakdown. Accordingly, there is problem that, when a reverse bias is applied to the p-n junction, the electric field will be readily concentrate in the circumferential part of the diffused region, resulting in an earlier breakdown than in central part of the diffused region, which is closer to the light receiving area. This phenomenon is known as edge breakdown. To prevent this edge breakdown, there are proposed an structure in which the circumference of the diffused region is surrounded by a guard-ring of a linearly graded p-n junction, whose breakdown voltage is relatively high and other structure having a partial charge sheet.
FIGS.1 to 3 are cross-sectional views showing prior art APDs for preventing edge breakdown.
FIG. 1 is a cross-sectional view of an APD having a partial charge layer. Referring to FIG. 1, an u-InGaAs absorption layer 2 and an u-InGaAsP grading layer 3 are successively formed on one surface of a n.sup.+ -InP substrate 1. On the u-InGaAsP grading layer 3 is formed a partial charge sheet layer 4 in order to prevent edge breakdown. The partial charge sheet layer 4 is thicker in the central part than in the other parts. An u-InP multiplying layer 5 is formed on the partial charge sheet layer 4 and a p.sup.+ -InP diffusion layer 6 is formed in the central part of the u-InP multiplying layer 5. To expose the p.sup.+ -InP diffusion layer 6, a passivation layer 7 is formed on the u-InP multiplying layer 5 and a p-type ohmic contact layers 8 are formed at both sides on the surface of the p.sup.+ -InP diffusion layer 6. A n-type ohmic contact layer 9 is formed all over the lower face of the n.sup.+ -InP substrate 1.
FIG. 2 is a cross-sectional view of an APD having floating guard-rings. Referring to FIG. 2, an u-InGaAs absorption layer 12 and a grading layer 13 are successively formed on an InP substrate 11. Then, a charge sheet layer 14 is formed on the surface of the u-InGaAsP grading layer 13. On the surface of the charge sheet layer 14 is formed an u-InP multiplying layer 15 and within the central part of the u-InP multiplying layer 15 is formed a p.sup.+ -InP diffusion layer 16. Then, a plurality of guard-rings 17a and 17b are formed such that they are separated from the p.sup.+ -InP diffusion layer 16 within the u-InP multiplying layer 15 and that they are separated from each other. On the surface of the u-InP multiplying layer 15 is formed a passivation layer 18 such that the p.sup.+ -InP diffusion layer 16 is exposed. On both sides of the surface of the p.sup.+ -InP diffusion layer 16 are p-type ohmic contact layers 19 which make ohmic contact with the p.sup.+ -InP diffusion layer 16. A n-type ohmic contact layer 20 which make ohmic contact with the n.sup.+ -InP substrate 11 is formed all over the lower face of the n.sup.+ -InP substrate 11.
FIG. 3 is a cross-sectional view of an APD having a guarding-ring. Referring to FIG. 3, on the surface of a n.sup.+ - InP substrate 21 are sequentially formed an u-InGaAs absorption layer 22 and a grading layer 23. A charge sheet layer 24 is formed on the surface of the u-InGaAsP grading layer 23 and an u-InP multiplying layer 25 is formed on the charge sheet layer 24. Then, within a central part of the u-InP multiplying layer 25 is formed a p.sup.+ -InP diffusion layer 26. To prevent edge breakdown, a guard-ring 27 is formed along the peripheral portion of the p.sup.+ -InP diffusion layer 26. On the surface of the u-InP multiplying layer 25 is formed a passivation layer 28 such that the p.sup.+ -InP diffusion layer 26 is exposed. Ohmic contact layers 29 which make ohmic contact with the p.sup.+ -InP diffusion layer 26 are formed on both sides of the surface of the p.sup.+ -InP diffusion layer 26. A n-type ohmic contact layer 30 is formed all over the lower face of the n.sup.+ -InP substrate 21.
As shown in FIGS. 1 to 3, a prior art APD comprises the partial charge sheet layer 14, and the floating guard-rings 17a and 17b. Alternatively, the prior art APD comprises a partial charge sheet layer 14 and a guard-ring 27. Such the partial charge sheet layer 14, the floating guard-rings 17a and 17b and the guard-ring 27 absorb an electric field at the edge portion of a pn junction face which is a junction face between the u-InP multiplying layer 5, 15, or 25 and p.sup.+ -InP diffusion layer 6, 16, or 26 and make an electric field higher in the central portion of the pn junction than in the edge portion thereof. Therefore, an avalanche breakdown occurs effectively at the central portion of the pn junction.
However, there are the following problems in the prior art APD as described above.
In APD having a partial charge sheet layer (refer to FIG. 1), the charge sheet layer is grown on the u-InGaAsP grading layer 3 by epitaxial growth and etched to form the partial charge sheet layer 4. Then, as the u-InP multiplying layer 5 is again grown on the partial charge sheet layer 4 by epitaxial growth, two epitaxial growth processes are required. Due to such two epitaxial growth, a defect at the interface between the partial charge sheet layer 4 and u-InP multiplying layer 5 occurs and the characteristics of APD thus decrease. Also, at the step of etching the charge sheet layer, it is difficult to accurately control the etching depth of the charge sheet layer.
In APD having a floating guard-ring (refer to FIG. 2), to form two floating guard-rings 17a and 17b, two or more diffusion processes are performed. At this time, as it is difficult to control diffusion process, it is hard to obtain APD having the superior characteristics.
In APD having the guard-ring as shown in FIG. 3, to form the guard-ring 27, two diffusion processes are performed. That is to say, after the first diffusion process utilizing Zn and Cd as diffusion source is performed, the second diffusion process utilizing Be as diffusion source is performed. At this time, as the exact doping control is difficult, there occurs the difficulty in the process.